1. Field of the Invention
This invention relates to a laser beam recording system, and more specifically to an optical system for deflecting a laser beam to make it scan a recording medium of small area such as a microfilm. The laser beam recording system in accordance with the present invention is particularly applicable to a high speed information recording system used for recording computer output information on a microfilm.
2. Description of the Prior Art
In view of the high power of the laser beam which is able to record information on a recording medium of low sensitivity, it has been proposed and practiced to use a laser beam in a high speed information recording system in which information is recorded at a high speed on a microfilm. In the laser beam recording system, a laser beam deflecting means is imployed to make the laser beam scan the microfilm.
As for the conventional laser beam deflecting means, a beam deflector employing an acousto-optic effect, a beam deflector employing an electro-optic effect, a beam deflector using a rotary polygonal optical element such as a rotary polygonal mirror or a rotary polygonal prism, and a beam deflector using a vibrating mirror such as a galvanometer are known and have been used.
The laser beam deflectors employing the acousto-optic effect or the elector-optic effect are advantageous in that it is capable of performing random access. However, such types, of beam deflectors are disadvantageous in the great loss of light and the low resolution. The laser beam deflectors using the vibrating mirror is unsuitable for performing a high speed scanning and a sweep of sawteeth waves and for recording images of large size, whereas this type of beam deflectors is advantageous in that the size thereof is compact and the driving circuit therefor is simple in construction. In view of these drawbacks of the above mentioned type of the beam deflectors, the beam deflectors using a rotary polygonal optical element are preferably used in the laser beam recording system.
Even when the beam deflectors using a rotary polygonal optical element are used, however, it is very difficult to record information on a minute recording medium. The beams incident to the deflector from different directions must be directed toward the same scanning line on the small recording medium. This is a great disadvantage of the laser beam recording system, in comparison with the image recording system in which an image on a cathode ray tube is focused on a recording medium using a lens system.
Further, the beam deflectors using a rotary polygonal optical element are disadvantageous in the following two points.
First, the rotary polygonal optical element such as a rotary polygonal mirror or prism requires a highly accurate manufacturing technique in processing the surface of the element and in mounting it to the shaft of a driving motor. All the reflecting or transmitting faces of the optical element must be precisely processed to be in parallel with each other. The optical element must be mounted on the shaft of a driving motor with the faces thereof oriented precisely in parallel to the shaft of the motor. Further, the angles formed between the adjacent faces of the optical element must be precisely processed to be equal to each other. In order to meet these requirements, a highly precise manufacturing technique is required.
Second, in order to form a sharply focused light beam spot by use of the polygonal optical element, a complicated lens system comprising a plurality of lenses of large diameter is required. In the laser beam recording system, the laser beam is required to be converged to a minute spot of the diameter of several microns. The minimum diameter .delta. theoretically derived from the diffraction limit is represented by a formula; .delta. = 1.27f.lambda./D, where f is the distance from the point of convergence to a converging lens, .lambda. is the wavelength of the laser beam and D is the diameter of the laser beam at the aperture of the converging lens. Accordingly, the distance f is desirable to be as short as possible and the diameter D is desirable to be as large as possible to form a minute beam spot. Therefore, the lens system for converging the laser beam to a minute beam spot is required to have a large diameter and a short focal length. However, the lens system which meets such requirements has a number of faces, which lower the power of the laser beam by repeating the reflection thereby.